The present invention relates to Tantalum and Niobium pellets used in the manufacture of Tantalum and Niobium capacitors. In particular, the invention is an improved method of sintering, deoxidization and doping with Nitrogen of Tantalum and Niobium pellets. Hereafter, the expression xe2x80x9cTantalum (Niobium)xe2x80x9d will mean xe2x80x9cTantalum or Niobiumxe2x80x9d.
This invention relates to an improved method of making Tantalum and Niobium pellets, and more particularly to the production of such pellets for use in electrolytic capacitors.
The usual method for making Tantalum (Niobium) pellets for use in Tantalum and Niobium capacitors includes steps wherein Tantalum (Niobium) powder is first pressed or compacted into pellets. The resulting pressed pellets then undergo a sintering process wherein the pellets are heated in a vacuum. The heating allows the Tantalum (Niobium) particles to stick together providing mechanical strength and electrical conductivity to the pellet.
The Tantalum (Niobium) lead xe2x80x9cwiresxe2x80x9d can be embedded into the pellets during the pressing process. During sintering, the Tantalum (Niobium) particles stick to the lead wire providing strong lead to pellet attachment. Such pellets are herein referred to as xe2x80x9cembedded type pelletsxe2x80x9d. The Tantalum (Niobium) pellets can be also pressed without lead wires. In that case, the pressed pellets undergo an initial sintering followed by lead wire to pellet welding. Such pellets are referred to here as xe2x80x9cwelded type pelletsxe2x80x9d. To purify the welding zone, the welded pellets undergo a second sintering process wherein the pellets with welded lead wire are heated in vacuum. This heating removes contamination from the welding zone by either evaporation of the impurities or redistribution of the impurities through out the pellet.
Following the sintering process, the Tantalum (Niobium) pellets are electrically anodized in an acid solution to form a dielectric film on the outer surface of the metal particles which is typically Tantalum (Niobium) Pentoxide. The pellet is subsequently coated with either various metal-containing materials (typically manganese dioxide) or conductive polymers, which form the cathode layer of the capacitor.
For electrolytic capacitors, oxygen concentration in the sintered Tantalum (Niobium) pellets is critical, especially when this concentration approaches the solubility limit of oxygen in the metal. Oxide phase precipitates on the surface of the Tantalum (Niobium)pellets act as efficient crystallization nuclei in the amorphous Tantalum (Niobium) Pentoxide film formed by anodization. The growth of these nuclei in the amorphous matrix of the Tantalum (Niobium) oxide films results in disruptions in the film, which cause degradation and failure in the electrolytic capacitors. The degradation rate is higher in Niobium capacitors than in Tantalum capacitors because the amorphous Pentoxide of Niobium is more susceptible to crystallization than the amorphous Pentoxide of Tantalum.
Besides crystallization, high oxygen concentrations in sintered pellets cause embrittlement of the lead wire in the vicinity of the sintered pellet. This is due to oxygen diffusion from the pellet into the lead wire; mostly into its crystalline grain boundaries. The brittle lead wire can be cracked or broken at assembly affecting capacitor yields and reliability.
Tantalum (Niobium) powders have a great affinity for oxygen and they may be significantly contaminated with oxygen during sintering. The major source for oxygen is the natural oxide film that always exists on the surface of Tantalum (Niobium) pellets when they are exposed to air. During heating in vacuum the oxygen from the surface oxide dissolves in the bulk of the Tantalum (Niobium) particles resulting in an increased concentration of oxygen in the sintered pellets.
Contamination with oxygen is a major barrier to the usage of high CV powders for Tantalum (Niobium) capacitors. This is also the major reason for the reduction of rated voltages in high CV capacitors. The higher the rated voltage, the thicker is the amorphous Tantalum (Niobium) Pentoxide film and therefore the film is more susceptible to crystallization.
A prior art method for reducing of oxygen concentration in sintered Tantalum (Niobium) pellets comprises the addition of oxygen active metals (preferably magnesium) to the sintered pellets and heating at temperatures above the melting point of the oxygen active metal and below the temperatures conventionally used for sintering the Tantalum (Niobium) pellets. During heating, atoms of the oxygen active metal react with oxygen in the Tantalum (Niobium) forming metal oxide molecules that evaporate out of the pellets. This reduces the oxygen concentration in the Tantalum (Niobium) pellets.
The above deoxidizing process has been used for welded type pellets after their initial sintering. Due to this process, the oxygen concentration in the initially sintered Tantalum (Niobium) pellets returns to the low level inherent in the Tantalum (Niobium) powder prior to sintering. However, with subsequent sintering the oxygen concentration in the Tantalum (Niobium) pellets increases again. This is due to dissolving of the natural oxide that forms on the Tantalum (Niobium) surface when pellets are exposed to air following the deoxidization process. The higher powder CV (surface area), the greater the oxygen concentration in the finally sintered pellets.
The above deoxidizing process is not used for embedded type pellets because it results in detachment of the lead wire from the pellet. The pull out strength for the lead wire drops from 2-4 kg after sintering to 0.1-0.2 kg after deoxidization. The electrical resistance of the lead to pellet junction increases sharply which makes impossible the anodizing of such pellets. As deoxidizing can""t be used, sintered embedded type pellets, especially very high CV pellets, are characterized by high oxygen concentration.
Another prior art method for reducing the oxygen concentration in Tantalum (Niobium) pellets involves Nitrogen doping of sintered pellets by means of heating in a nitrogen containing atmosphere, (preferably Nitrogen gas) so that diffusion of nitrogen into the Tantalum (Niobium) particles occurs. Nitrogen doping prevents diffusion of oxygen from the ambient atmosphere and from the anodic oxide film into the Tantalum (Niobium) particles resulting in stabilization of the amorphous structure and chemical composition of the dielectric film. As a consequence, Tantalum (Niobium) capacitors made from such pellets have low and stable direct current leakage (DCL) and high reliability.
Nitrogen doping of Tantalum (Niobium) pellets requires a low oxygen concentration in the sintered pellets. Both impurities (Nitrogen and Oxygen) occupy the same positions in the crystalline lattice of Tantalum (Niobium). When these positions have been occupied by oxygen, diffusion of Nitrogen into the Ta (Nb) becomes virtually impossible. For this reason, sintered Tantalum (Niobium) pellets with high oxygen content can""t be properly doped with Nitrogen.
It is therefore a principal object of this invention to conduct a process for producing capacitor pellets continuously within an oxygen free environment.
It is a further object of the present invention to provide a method of reducing the oxygen content of sintered Tantalum (Niobium) pellets.
It is a further object of the present invention to provide a method of making flexible lead wires with strong attachment to the sintered pellet.
It is a further object of the present invention to provide a method of doping of sintered Tantalum (Niobium) pellets with Nitrogen
It is yet a further object of the present invention to provide low and stable (DCL), high CV, an extended range of rated voltages, and high reliability of the Tantalum (Niobium) electrolytic capacitors produced from Tantalum (Niobium) pellets sintered according to this invention.
These and other objects will be apparent to those skilled in the art.
This invention provides a process by which sintered Tantalum (Niobium) pellets are produced having low oxygen content, flexible lead wires with strong attachment to the pellets, and are doped with nitrogen. Before application of this process, Tantalum (Niobium) powder is pressed into the pellets with embedded lead wires or without lead wires. In the last case, the pressed pellets undergo an initial sintering in vacuum and lead wire to pellet welding. The new process includes three steps running one after another and without exposure of the pellets to an oxygen-containing environment. The first step is deoxidizing of the Tantalum (Niobium) pellet by an oxygen active metal (preferably magnesium) previously added to the pellets. The second step is annealing of the pellets in either inert gas or vacuum in the temperature range from that conventionally used for deoxidizing to that conventionally used for sintering. This process step provides either sintering of the embedded pellets or purification of the welding zone in welded pellets. The third step is doping of the sintered pellets with nitrogen by heating in a nitrogen-containing atmosphere (preferably nitrogen gas) at temperatures lower than the annealing temperature. Usage of Tantalum (Niobium) pellets processed continuously within an oxygen free environment according to this invention allows one to produce Tantalum and Niobium capacitors with high CV, an extended range of rated voltages, low and stable DCL, and high reliability.